The present invention relates in general to electrographic formation of toner powder images on a recording medium and more specifically, to a toner powder transport system for providing controlled movement of magnetically attractable toner powder from a toner powder reservoir to a recording area at which an imaging styli array is positioned and providing for the removal of unused toner powder from the recording area plus its return to the toner powder reservoir.
As indicated in U.S. Pat. No. 3,946,402 to Lunde, there are prior art electrographic recording systems for forming toner powder images on a recording medium which provide for the transporting of toner powder to a recording region in some manner. Various types of toner transport systems have been devised. U.S. Pat. No. 3,121,375 to Fotland et al., U.S. Pat. No. 2,932,548 to Nau et al discloses arrangements wherein toner powder is provided to the recording region through a portion that serves as a recording electrode. This approach for supplying toner powder to the recording region is not practical where the recording electrodes are spaced very close to each other.
Another arrangement for delivering toner powder to a recording region involves use of a separate developing element to which toner is first applied. The developing element may be in the form of a porous endless belt on which the toner is carried into the recording region, as disclosed in U.S. Pat. No. 3,355,743 to Capps, or may in the case of magnetically attractable toner powder, be in the form of a cylinder that encloses a magnet assembly that presents alternate magnetic poles adjacent the cylinder wherein rotation of the magnet assembly or cylinder brings the toner powder to the recording region as taught in U.S. Pat. No. 3,816,840 to Kotz.
A primary deficiency with the above types of prior art toner applicators is that they do not provide adequate control of the amount and movement of toner powder to the recording region and removal of unused toner from the recording region when large numbers of narrowly spaced apart imaging styli are employed in an imaging styli array. It is difficult to precisely meter thin, uniform amounts of toner powder into the recording region without experiencing occasional plugging of metering orifices or doctor blade gaps because of toner powder agglomeration. If too much toner powder is transported into the recording region, the toner powder images formed tend to bloom out and will not have sharply defined edges. When plastically deformable toner powders are used, an excessive amount of toner powder in the recording region may increase pressure applied on the toner powder by the recording electrode and the recording medium to the point that there is a gradual toner build-up on the recording electrode. If inadequate toner is transported onto the recording region, the formed toner images suffer a loss of density and become light or disappear altogether.
U.S. Pat. No. 3,946,402 to Lunde discloses a toner applicator arrangement designed to overcome the deficiencies of the above described systems which accurately meters an optimum level of toner from a toner supply onto a recording member which moves to present the toner powder to a recording region in order that high quality, high resolution toner images can be formed. Nonrecorded or unused toner powder is magnetically removed from the recording member and is then physically removed by a remover after allowing the removed toner to fall by gravity back to the toner supply. With this arrangement an undesirable amount of toner powder is projected into the air and also excess toner powder in the removal region becomes air entrained. In addition, since the recording member is pre-toned by this arrangement, considerable background toner powder remains on the recording member, particularly on rough recording members such as rough paper.